This invention relates generally to semiconductor structures and manufacturing methods and more particularly to alignment techniques used therein.
As is known in the art, semiconductor integrated circuits are manufactured using a series of process steps which require proper alignment of the semiconductor wafer. Many alignment systems use reflected light from profile patterns formed on the surface of the semiconductor wafer to determine the location of the wafer. Such an arrangement is shown in FIG. 1. An alignment illumination 10, here a cross, is focused onto the surface 12 of the semiconductor wafer 14 using an optical system 16. A portion of the light is reflected from the surface of the semiconductor wafer is directed by the optical system 16 to a detector arrangement 20. The wafer 14 has formed along one portion thereof an alignment mark 22, here shown diagrammatically as a series of grooves 24 etched into the surface 12 of the wafer 14. As the wafer 14 is scanned horizontally, the detector arrangement 20 produces waveforms which enable detection of the alignment of the wafer 14 relative to the optical system 16.
More particularly, and referring also to FIG. 2, there are shown four sites, i.e., site 1, site 2, site 3 and site 4 of alignment marks on each of both the upper and lower peripheral portions of a semiconductor wafer 14. Each one of the sites includes two sets of lines 13, one at +45 degrees with respect to the vertical, or Y axis, and the other set of lines 15 being at −45 degrees with respect to the Y axis. The alignment illumination projected by the optical system (FIG. 1) onto the surface of the wafer is a cross, such as used in the MICRASCAN equipment manufactured by Silicon Valley Group (SVG), San Jose, Calif. A “standard” alignment mark, in one half of a site, for the MICRASCAN III equipment is shown in FIG. 3 and consists of wide stripes at a 45 degree angle separated by variable spacing. Another version is shown in FIG. 4 and is made up of lines at the locations where the “standard” mark has the edges of its stripes. The size of both versions is 60×60 micrometers. The alignment marks etched into the surface of the wafer are shown in FIG. 2 as a pair orthogonal sets of a series of parallel lines, only one of the two sets being shown in FIGS. 3 and 4.
Referring again to FIG. 1, the alignment illumination, a cross is projected onto the surface 12 of wafer 14 with the pair of intersecting arms of the cross being disposed nominally orthogonal to the lines in each of the sites. The cross-shaped light (i.e., the alignment illumination) is projected by the optical system 16 onto, and scanned across the site (FIG. 2) along the X direction indicated on the surface 12 of the wafer 14. The optical system 16 includes a prism (FIG. 1) which directs a portion of the light reflected surface 12 of the wafer 14 onto a detector arrangement 20 shown diagrammatically in FIG. 1. Thus, as indicated, there are four detectors 221, 222, 223, and 224; one pair 221 and 222 being disposed along an axis +45 degrees with respect to the Y axis and one pair 223 and 224 being disposed along an axis −45 degrees with respect to the Y axis. The pair of detectors 221 (i.e., “Left +45”) and 222 (i.e., “Right +45”) is used for detection of light reflected by lines 13 at +45 degrees with respect to the Y axis and the pair of detectors 2223 (i.e., “Left −45”) and 224 (i.e., “Right −45”) are used to detect light reflected by lines 15 at −45 degrees with respect to the Y axis.
More particularly, to determine the location of an alignment site, two marks 13, 15 (FIG. 1); one oriented at +45 degrees and one at −45 degrees with respect to the Y axis, are required. The alignment marks 13, 15 are scanned by the optical system with an X shaped illumination, as described above. The light reflected from the surface of the wafer and the alignment lines is detected in the dark field mode, i.e., only light scattered from the marks at an angle is analyzed. Two detectors 221 and 222 record simultaneously the reflected light; one detector 222 located to the right side and one detector 221 to the left side of the mark's edge. When scanning the +45 degree lines 13, the set of detectors 221 and 222 is activated and when the −45 degree lines 15 are scanned, the set of detectors 223, 224 are activated. More particularly, referring to FIG. 1, when the alignment illumination is over the +45 degree lines 13 of site 1, the “Left +45” and “Right +45” detectors 221 and 222 are activated and the “Left −45” and “Right −45” detectors 223 and 224, are deactivated. When the alignment illumination moves over the −45 degree lines 15 of site 1, the “Left −45” and “Right −45” detectors 223 and 224 are activated and the “Left +45” and “Right +45” detectors 221 and 222 are deactivated. It is noted that with such an arrangement, each alignment site is made up of a pair of spatially separated sets 13, 15 of parallel orthogonal lines with two sets in the site being sequentially activated/deactivated detectors. Such spatial separation increases the area required for an alignment site.